Photocatalytic degradation activity of goji berry extract synthesized silver-loaded mesoporous zinc oxide (Ag@ZnO) nanocomposites under simulated solar light irradiation

Different approaches have been developed for the synthesis of various nanostructured materials with unique morphologies. This study demonstrated the photocatalytic and antimicrobial abilities of silver-loaded zinc oxide nanocomposites (Ag@ZnO NCs). Initially, ZnO with a unique mesoporous ellipsoidal morphology in the size range of 0.59 ± 0.11 × 0.33 ± 0.09 µm (length × width) was synthesized using aqueous precipitation in a mild hydrothermal condition (80 °C) with the aqueous fruit extract of goji berry (GB) (as an additive) and calcined in air at 200 °C/2 h and 250 °C/3 h. Powder X-ray diffraction (XRD) revealed the formation of a hexagonal phase of the wurtzite (WZ) structure. The average crystallite size of ZnO was 23.74 ± 4.9 nm as calculated using Debye–Scherrer’s equation. It also possesses higher thermal stability with the surface area, pore volume, and pore size of 11.77 m2/g, 0.027 cm3/g, and 9.52 nm, respectively. Furthermore, different mesoporous Ag@ZnO NCs loaded with face-centered cubic (fcc) silver nanoparticles (Ag NPs) in the range of 90–160 nm were synthesized by GB extract as a reducing and capping agent on the surface of ZnO after calcination in air. The immobilization of Ag NPs was confirmed by XRD, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), FE-transmission electron microscopy (FE-TEM), and energy-dispersive X-ray spectroscopy (EDS). It was found that Ag0.2@ZnO NC (0.2 wt% of Ag) showed excellent photocatalytic degradation of both methylene blue (MB) (cationic) and congo red (CR) (anionic) dyes under simulated solar irradiation. The photocatalytic degradation of 99.3 ± 0.35% MB and 98.5 ± 1.3% CR occurred in 90 and 55 min, respectively, at room temperature by Ag0.2@ZnO NC. Besides, these NCs also showed broad-spectrum antibacterial activity against both Gram-positive and Gram-negative bacteria. The mechanistic concept of generating reactive oxygen species (ROS) by electron and hole charge (e‾/h+) carriers seems to be responsible for the photocatalytic degradation of commercial dyes and antibacterial activities by Ag@ZnO NCs. Thus, these silver-loaded mesoporous ellipsoidal ZnO NCs are promising candidates as photocatalysts for industrial/wastewater treatment as well as in antimicrobial therapeutics.

Preparation of goji berry (GB) extract. The aqueous extract of goji berry (GB) (Lycium barbarum L.) fruit was done as mentioned earlier 30 . Briefly, the dried GBs were chopped into small pieces and then excellently ground into a coarse powder in a mortar pestle. The aqueous extract was prepared by heating 5.0 g of GB powder in 100 mL of deionized water taken in a 250 mL Erlenmeyer flask and allowed to boil with stirring at 100 °C for 15 min. Later, the solution was cooled to room temperature and centrifuged at 4000 rpm for 10 min, and filtered through Whatman No. 1 filter paper to obtain a clarified solution of GB extract. Finally, the aqueous GB extract was stored in the refrigerator at 4 °C for the preparation of metal nanoparticles and metal/semiconductor nanocomposites. Initially,8.0 g zinc nitrate hexahydrate was dissolved in 100 mL deionized water and stirred at room temperature for 5 min. Then, 30 mL of freshly prepared GB extract solution was added, and the pH of the solution was adjusted to 9.0 with the dropwise addition of aqueous NH 4 OH. The resultant mixture was continuously stirred under a mild hydrothermal condition of 80 °C for 24 h. The obtained yellow precipitate was collected by centrifugation at 4000 rpm for 15 min and washed twice with deionized water. The as-prepared zinc oxide (ZnO) particles were dried in a vacuum oven at 60 °C overnight, followed by calcination at 200 °C/2 h and 250 °C /3 h in air and stored in an airtight amber vial.

Preparation of silver-loaded zinc oxide nanocomposites (Ag@ZnO NCs). To prepare different
Ag@ZnO NCs, 6.0 g of ZnO was added with varying quantities of silver (0.2%, 0.4%, and 0.8% (w/v)) in 100 mL of deionized water taken in an amber bottle. The solution was sonicated for 30 min to homogeneous the solution containing silver nitrate and ZnO. Then, 40 mL of GB extract was added dropwise with constant stirring at 60 °C for 3 h. The formed precipitate was washed three times with deionized water after centrifuging at 10,000 rpm for 20 min and dried in a hot air oven at 60 °C overnight. These dried nanocomposite powders were calcined in air at 200 °C/2 h and 250 °C/3 h and stored in an amber vial for further experiments. These nanocomposites with different silver concentrations of 0.2%, 0.4%, and 0.8% (w/v) were referred to as Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs, respectively. The pictorial representation of the synthesis of these metal/semiconductor nanocomposites is provided as the schematic diagram (Fig. 1).

Characterization of ZnO and Ag@ZnO NCs. The optoelectronic properties of ZnO and different Ag@
ZnO NCs were determined from the ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) recorded using a VARIAN Cary 5000 spectrophotometer (Agilent Technologies, USA) equipped with a Praying Mantis diffuse reflectance accessory (DRA). Powder X-ray diffraction (XRD) analysis was performed to determine www.nature.com/scientificreports/ the crystalline structure of the nanocomposites using a PANalytical X'PertPRO MPD (Netherlands) X-ray diffractometer with Cu Kα1 radiation (0.15406 nm) and operating conditions of tube voltage 40 kV, tube current 30 mA, and scanning between 7.0° and 80.0° (2θ) at a rate of 1.2°/min. The diffraction peaks of the crystalline phases were compared with the standard compounds of the JCPDS data. The average crystallite size (D) of the samples was calculated using the Debye-Scherrer's equation: D = Kλ/βcosθ, where K is Debye-Scherrer constant (0.89), λ is the X-ray wavelength (0.15406 nm), β is the full-width at half maximum (FWHM), and θ is the diffraction angle. Fourier-transform infrared (FTIR) spectroscopy was performed using a Perkin-Elmer FTIR (Model: Spectrum 100) spectrometer in transmittance mode with the wavenumber range of 400-4000 cm −1 .
The hydrodynamic size and zeta potential of the samples were analyzed using a Zetasizer nanoparticle analyzer (Malvern Instruments Worc, UK; Model: ZS90) at 25 °C 31 .
To analyze the morphology and composition of the nanocomposites, field-emission scanning electron microscopy (FE-SEM) (Hitachi, Japan; Model: S-4200) was performed by mounting the samples on an aluminum stub and sputter-coated with platinum and analyzed with secondary electron (SE) detectors at operating voltages of 10 and 15 kV. The elemental composition was analyzed by SEM-energy dispersive X-ray spectroscopy (SEM-EDX). The shape and size of the nanocomposites were examined using an FE-transmission electron microscope (FE-TEM, FEI Tecnai G2 F20, Oregon, USA) at an accelerating voltage of 200 kV. The elemental analysis of the nanocomposites was also analyzed using the high-angle annular dark-field scanning TEM energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS). The oxidation state of each element of the nanocomposite was analyzed using X-ray photoelectron spectroscopy (XPS) via a Thermo Scientific K-Alpha system with an Al Kα X-ray source and the ion source energy was between 100 V and 3.0 keV for the survey 30 . The thermal stability of nanocomposites was analyzed by thermogravimetric analysis (TGA) from room temperature to 800 °C at a heating rate of 20 °C/min in a nitrogen atmosphere. Photoluminescence (PL) spectroscopy was performed using the HORIBA Scientific Raman system and analyzed with LabSpec 6 software. A 325 nm air-cooled He-Cd laser power at 50 mW with Syncerity CCD detector was used and detected with 10 × objective in the wavelength range of 340-1050 nm. Brunauer-Emmett-Teller (BET) surface area (S BET ), Barrett-Joyner-Halenda (BJH) pore size distribution and pore volume of samples were analyzed using a Micromeritics 3Flex adsorption analyzer (Norcross, GA, USA). The photocatalytic degradation of dyes was evaluated using a Shimadzu UV-2600 dual-beam UV-Vis spectrophotometer (Kyoto, Japan).

Applications of ZnO and Ag@ZnO NCs
Photocatalytic degradation of dyes. The photocatalytic degradation of dyes (MB and CR) by ZnO and Ag@ZnO NCs as photocatalysts was assessed by the decolorization of dye solutions with the initial concentrations of 10 mg/L MB or 20 mg/L CR under simulated solar light irradiation using Ultra-Vitalux lamp (300 W) (Osram GmbH). In the photocatalysis, 0.1% (w/v) of ZnO and various Ag@ZnO NCs were taken as photocatalysts and added to 100 mL of aqueous dye solutions under continuous stirring. Before simulated solar irradiation, the dye solution with photocatalyst was incubated at room temperature in the dark for 30 min to reach adsorption-desorption equilibrium. The distance between the lamp and the dye solution was kept at 10 cm, and the samples were taken periodically and centrifuged at 12,000 rpm for 10 min to remove the nanocomposites from the dye solutions. The maximum absorbance (λ max ) of the supernatant dye solution was analyzed by a dual-beam UV-Vis spectrophotometer to quantify the concentrations of MB and CR dyes at 663 and 498 nm, respectively. The rate of degradation of dyes was calculated by the percentage of the concentration of dye that remained after a specific time over the initial dye concentration.
where C 0 and C t are the initial and final concentrations of dyes at a reaction time (t), respectively. Antibacterial assay. The antibacterial activity of ZnO and Ag@ZnO NCs was tested against both Gramnegative (E. coli) and -positive (S. aureus) bacteria using the agar well diffusion method 32 . The overnight cultures of E. coli and S. aureus were obtained by inoculating the MH broth with the pure single colonies of bacteria. Later, the MH agar plates were spread-plated with pure bacterial suspensions, and the agar wells were made using a sterile cork borer with a diameter of 8 mm. Different Ag@ZnO NCs and ZnO (2 mg; 40 mg/mL) were loaded into the wells, and the plates were incubated at 37 °C for 16 h. Ampicillin (300 µg for S. aureus, and 500 µg for E. coli) was used as a positive control. The development of the zone of inhibitions (ZOIs) around the ZnO and Ag@ZnO NCs loaded wells was measured and recorded.

Results and discussion
Synthesis of ZnO and Ag@ZnO NCs. Generally, plant/fruit extracts have great potential in the synthesis of nanoparticles and nanocomposites. The aqueous extract of goji berries contains several phytochemicals such as phenylpropanoids, coumarins, lignans, and isoflavonoids providing natural reduction, capping, and/or stabilization moieties over the expensive chemicals to form metal nanoparticles and nanocomposites 30 . During the synthesis of ZnO by direct precipitation method with GB extract, the color of the solution changed to light yellowish and precipitated within 30 min at 80 °C (pH 9.0), indicating the formation of ZnO. ZnO nanostructure was synthesized by a simple precipitation method with the addition of ammonium hydroxide (NH 4 OH) as an oxidizer in the presence of GB extract as an additive 33 . The formation of unique morphology is perhaps the only challenge of the precipitation method. The use of several additives in precipitation aqueous solution catalyzes and functions as a morphology directing agent for the formation of unique morphology. The biomacromolecules and metabolites of GB extract direct the formation of unique ZnO nanostructures. Qi et al. 34 reported that dextran promoted the formation of flower-like ZnO nanostructure, and positively charged homopolymer, www.nature.com/scientificreports/ poly-l-Lysine (PLL) as an additive was reported to catalyze the formation of ZnO formation 35 . The excess of bioorganic components attached to ZnO particles from the GB extract could have been decomposed in the calcination process in the air releasing ZnO particles with unique morphology 36,37 . Sadiq et al. 19 demonstrated the synthesis of ZnO NPs using the leaf extract of Syzygium cumini (black plum). Besides secondary metabolites, plant/fruit extracts also contain many biomolecules such as proteins, polysaccharides, terpenoids, and alkaloids that could have been involved in the bioreduction and stabilization of various metal/metal oxide nanoparticles and nanocomposites 38 . When different concentrations of silver (0.2-0.8% w/v) were mixed with ZnO suspension and subsequently with GB extract, the solution color changed to light greenish, denoting the formation of Ag NPs on the surface of ZnO as Ag@ZnO NCs at 60 °C within 3 h (Fig. 1). The optical properties of the colloidal solution depend on the nanoscale morphology as well as the distance between them 39 . It has been postulated that the keto-enol tautomeric transformation of polyphenolic compounds of fruit extract such as flavonoids may release the reactive hydrogen atoms, which drive the reduction of Ag ions and enable the formation of Ag NPs [40][41][42] . In addition, the internal conversion of ketones to carboxylic acids in flavonoids was also likely to be involved in the reduction process of silver ions to Ag NPs 43 .

Characterization of ZnO and Ag@ZnO NCs. The optical properties of the ZnO and Ag@ZnO NCs
were investigated by the UV-Vis diffuse reflectance absorption spectra. Figure 48 demonstrated the synthesis of ZnO NPs with hexagonal wurtzite crystal structure from the Sedum alfredii Hance, a Zinc hyperaccumulating plant. In another instance, a bio-based approach was used to synthesize crystalline and polydispersed ZnO NPs (72.5 nm) using Physalis alkekengi L. to remediation of zinc-contaminated soils 49 . The average crystallite size of ZnO calculated using the Debye-Scherrer's equation was 23.74 ± 4.9 nm. Also, the strong and sharp diffraction peaks confirm the high crystallinity of ZnO, and the degree of crystallinity was calculated through the equation: [area of crystalline peaks/area of crystalline & amorphous peaks] × 100, showed 100% crystallinity. Zaid et al. 50 reported that calcination at higher temperatures could improve the crystallinity and better particle distribution. In Ag@ZnO NCs, the additional peaks of 38.11°, 44.30°, and 64.45° correspond to (111), (200), and (220) planes of face-centered cubic (fcc) phase of silver (JCPDS card No. 2-109) 51 . The ionic radius of silver ion (Ag + ) (0.122 nm) was larger than that of zinc divalent (Zn 2+ ) ions, thus silver ions cannot be substituted into the crystal lattice of the ZnO matrix; therefore, the metallic silver peaks due to the Ag NPs are formed over the ZnO surface 45 . The FWHM and crystallite size are inversely proportional; therefore, the increase in the size of Ag NPs results in the formation of larger NCs. These Ag NPs formed on the surface of ZnO were in the size of 25.65 ± 5.0, 32.91 ± 3.3, and 33.32 ± 4.21 nm in diameter for Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs, respectively. The intensity of Ag NPs peaks increases with the increase in the silver content of NCs, which is due to the increase in the number of Ag NPs on the surface of ZnO.
The functional groups involved in the formation of ZnO and Ag@ZnO NCs were investigated by the Fouriertransform infrared (FTIR) spectroscopy in the range of 400-4000 cm -1 (Fig. 2d). FTIR spectra of all samples and GB extract exhibited various absorption bands. In GB extract spectrum, the broad band centered at 3290 cm -1 was assigned to hydrogen-bonded O-H stretching vibrations and the weak signal at 2936 cm -1 was due to C-H stretching vibrations 52 . The band at 1595 cm -1 was attributed to the C-OH deformation vibration and the band at 1417 cm -1 was due to the O-C-O symmetric and asymmetric stretching vibrations of the carboxylate group. Moreover, the band at 1025 cm -1 was assigned to C-O stretching vibrations of the pyranose ring 30,53 . The FTIR spectra of ZnO and Ag@ZnO NCs exhibited a difference from the GB extract spectrum, the intensity of the broad band around wavenumber 3396 cm -1 , the characteristic of OH stretching vibration, decreased in all samples after calcination 54 . Meanwhile, the broad absorption bands around 400-600 cm -1 were attributed to the stretching modes of metal-oxygen bonds, thus confirming the formation of Zn-O bonds 55 .
Dynamic light scattering (DLS) is a relatively robust and economical technique to measure the average size and size distribution of synthesized nanoparticles and nanocomposites. Mainly, DLS provides larger values because of the hydrodynamic shell, which is dependent on the structure, shape, and roughness of the particles 56 . According to Stokes-Einstein (SE) equation, the measured diffusion coefficients are related to the hydrodynamic radius as D = k B T/6πηR h , where k B is Boltzmann's constant (1.38 × 10 -23 J/K), T is the temperature, η is the viscosity of the suspension medium, and R h is the hydrodynamic radius 57 . There was an increase in the size of nanocomposites with the addition of silver to ZnO (Fig. 3a-d). The increase in the size can be caused by the www.nature.com/scientificreports/ formation of Ag NPs on the surface of ZnO particles and the aggregation of NCs. The zeta (ζ) potential is used to study the surface charges and stability of nano-or submicronic particles. The biomolecules from the GB extract were involved in reducing and stabilizing nanoparticles and nanocomposites. The zeta potential was calculated by dispersing the particles in water as the dispersion medium. The values of zeta potential were correlated with their stabilities: 0 to ± 5 (rapid coagulation), ± 10 to ± 30 (incipient stability), ± 30 to ± 40 (moderate stability), ± 40 to ± 60 (good stability), and > ± 61 (excellent stability) 58,59 . The average zeta potential of synthesized ZnO after calcination was + 2.72 mV indicating positively charged groups in the stabilization. However, the zeta potentials of Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs were − 16.4, − 28.1, and − 0.46 mV, respectively (Fig. 3e-h). This shows that with the increase in the formation of Ag NPs on the surface of ZnO, the stability of Ag 0.2 @ZnO and Ag 0.4 @ZnO NCs increases, whereas the stability decreases with Ag 0.8 @ZnO NC. The surface morphology of ZnO and various Ag@ZnO NCs were identified using FE-SEM. Before calcination, the morphology of ZnO synthesized by precipitation method using GB extract as an additive promoted the formation of spike-like spherical morphology, which was different from the plate-like morphology of ZnO synthesized by direct precipitation method without any additive (Supplementary Fig. 1). This indicates the role of GB extract as a sustainable and eco-friendly material directing the formation of unique ZnO morphology. However, after the calcination process, the morphology of ZnO particles with GB extract was rearranged as clusters of ellipsoids with slight polydispersity on a submicronic scale. The ellipsoidal particles were in the size of 0.7 ± 0.13 and 0.38 ± 0.075 µm (length × width) (Fig. 4a,b). Remarkably, all ZnO particles were almost identical in dimension, and the surface looks puffy with an irregular pattern of pillar ridges. There was a slight agglomeration of particles due to the slightly higher surface area and durable affinity among ZnO particles 58 . Different morphologies of ZnO NPs, for example, nanospheres, nanoflower, nanoflakes, nanobelt, nanorods, nanowires, nanoneedles, nanotubes, and nanorings, can be synthesized by controlling the synthesis parameters 24,60,61 . The addition of silver with GB extract formed spherical Ag NPs i.e., 0.06 ± 0.011, 0.09 ± 0.04, 0.14 ± 0.045 µm for Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs, respectively, on the surface of ZnO particles. There was no significant change in the morphology of Ag@ZnO NCs except with the size of embedded Ag NPs on the surface of ZnO, which increased with the increase in the silver content added to the NCs (Fig. 4c-h).
The elemental composition of ZnO and Ag@ZnO NCs was analyzed using the FE-SEM-EDX spectra, as shown in Fig. 5. The spectrum of ZnO particles showed a low energy peak at approximately 0.533 keV (O-Kα) due to the presence of oxygen atoms, and other peaks for zinc and carbon atoms appeared at about 1.02 keV (Zn-Lα), 8.6 keV (Zn-Kα), 9.5 keV (Zn-Kβ), and 0.285 keV (C-Kα). In contrast, Ag@ZnO NCs spectra contain intense low energy silver peaks at approximately 2.61 keV (Ag-Kα), 3.0 keV (Ag-Lα), 3.2 keV (Ag-Lβ), and 3.4 keV (Ag-Lβ2) along with Zn, O, and C peaks 62 . The EDX quantified silver, and other elements content in various Ag@ ZnO NCs is shown in Fig. 5 (inset). The weight percent of silver increases from Ag 0.2 @ZnO to Ag 0.8 @ZnO, which infers the successful incorporation of silver as Ag NPs on ZnO 63 . Hence, the weight percentage of silver loaded on ZnO is proportional to the Ag concentration added to ZnO in the preparation of different Ag@ZnO NCs.
FE-TEM analysis of ZnO and different Ag@ZnO NCs are shown in Fig. 6. Agglomeration of ellipsoidal ZnO submicronic particles and the formation of spherical Ag NPs on the ZnO surface were further verified by the FE-TEM results. It was found that ZnO was about 0.6 ± 0.11 and 0.33 ± 0.087 µm (length × width), whereas Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs have Ag NPs with the size of 87 ± 55, 130 ± 43 and 160 ± 55 nm, respectively. Furthermore, these results corresponded to the poor correlation between the FE-TEM sizes and particle size distribution analysis. With the increase in the silver content in the NCs, there was an occurrence of large particles due to the aggregation of small or primary particles. The 'd' spacing of ~ 0.281 nm between the adjacent lattice planes could be attributed to the (002) plane of ZnO ( Fig. 6-d3). Similarly, the lattice fringes with d = ~ 0.24 nm could be attributed to the (111) planes of Ag NPs ( Fig. 6-b,c3). All these results confirmed the successful formation of Ag NPs on the surface of ZnO. The d-spacing of the (002) plane of ZnO in Ag@ZnO NC is like that of undoped ZnO, suggesting that Ag + ions are not incorporated into the ZnO lattice. SAED pattern of ZnO clearly showed well-resolved diffraction rings indicating the polycrystallinity, and Ag NPs on ZnO (Ag@ ZnO NCs) showed bright spots indicating the monocrystalline nature. Figure 7 shows the HAADF-STEM image of the Ag 0.2 @ZnO NC and its corresponding elemental composition (Zn-K, Zn-L, O-K, Ag-K, and Ag-L) by STEM-EDX mapping. These images confirm the successful embedment of Ag NPs on the surface of ZnO.
XPS analysis was performed to clarify the chemical states of elements in Ag@ZnO NCs. The full scan survey of Ag 0.2 @ZnO NC shows the signals from Zn, O, and Ag elements with their corresponding atomic percent of 38.88, 37.34, and 1.63% in the range 0-1350 eV (Fig. 8a). Figure 8b shows the high-resolution spectra of Zn 2p. The peaks of Ag 0.2 @ZnO NC were located at 1021.28 eV and 1044.38 eV, which were ascribed to Zn 2p 3/2 and Zn 2p 1/2 , respectively. These peaks confirm that the Zn element exists in a divalent cation (Zn 2+ ) state in the NC. Figure 8c shows the high-resolution O 1 s peak of Ag 0.2 @ZnO NC. The deconvoluted O 1 s peak shows two subpeaks at binding energies of 529.8 and 531.2 eV attributing to the lattice oxygen of ZnO and dissociated oxygen or hydroxyl-like group on the surface of ZnO, respectively 64,65 . The presence of surface hydroxyl groups acts as adsorption sites of dyes and reacts with photogenerated holes forming hydroxyl radicals by oxidation, which decomposes dyes during photodegradation 66 . Therefore, the presence of a surface hydroxyl group with 28.9% was one of the critical factors in the photodegradation process. Figure 8d shows the high-resolution spectrum of Ag 3d photoelectron peaks of Ag 0.2 @ZnO NC. The Ag 3d spectrum shows two peaks centered at 367.38 and 373.48 eV ascribed to Ag 3d 5/2 and Ag 3d 3/2 transitions, respectively. The difference in the binding energy of ~ 6.0 eV between Ag 3d 5/2 and Ag 3d 3/2 peaks was the characteristic of metallic silver and consistent with the results of XRD analysis 30,67 . Figure 9a shows the thermogravimetric (TG) analysis of ZnO, Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs. TG analysis was performed from room temperature to 800 °C at a rate of 20 °C/min in a nitrogen atmosphere to demonstrate the thermal stabilities of ZnO and Ag@ZnO NCs. The overall weight loss for all samples was very  68 . This explains that ZnO absorbs nitrogen and slowly releases them over a period, which indicates that ZnO was pure and very porous in nature 69 . The weight loss accounting for ~ 2.1% from 350 to 700 °C was higher with ZnO; this could be due to the thermal decomposition of biomolecules of GB extracts, such as phenolic compounds and other metabolites. Above 700 °C, there was no significant weight loss in ZnO. A similar decomposition pattern was observed with Ag 0.2 @ZnO NC; however, the embedment of silver slightly improved the thermal stability and decomposition of Ag 0.2 @ZnO NC compared to that of ZnO. In Ag 0.4 @ZnO and Ag 0.8 @ZnO NCs, the continuous decrease in PL spectrum is a valuable tool to investigate the state of photogenerated e‾/h + pairs and the defects of metal/ semiconductor nanocomposites 70 . Figure 9b shows the PL spectra of ZnO and Ag@ZnO NCs at room temperature. There are two distinct emission peaks in the UV region (~ 380 nm) and visible region (~ 400 to 750 nm). These emission peaks provide information about the recombination between charge carriers and defect levels 21 . The emission peak at ~ 380 nm in ZnO corresponds to near band edge emission (NBE), attributed to bandgap excitation 71 . The broad band emission extending from ~ 400 to 750 nm in the whole visible spectrum can be from deep-level emission (DLE), i.e., because of crystal defects like Zn-interstitials and oxygen vacancies 55,72 . All Ag@ZnO NCs showed decreased PL intensity than ZnO, which suggests the decrease in the recombination rate of photoinduced electrons and holes with the embedment of Ag NPs favoring the photocatalytic activity than ZnO 73 . The PL intensity of Ag 0.2 @ZnO NCs decreased drastically with the increase in the silver content in the NCs providing the separation of photoinduced e‾/h + pairs and inhibiting the recombination of photoinduced pairs 74,75 . However, at Ag 0.8 @ZnO NC, with the increase in the Ag concentration, there was no increase in the PL intensity and overlapped with the peaks of Ag 0.4 @ZnO NC, suggesting the formation of new recombination centers, which are unfavorable to the separation of photoinduced pairs 70,76 . Thus, Ag 0.8 @ZnO NCs exhibited the lowest PL intensity as that of Ag 0.4 @ZnO NC because excess addition of silver as Ag NPs in Ag 0.8 @ZnO NC was unfavorable for charge separation.
To determine the structural and adsorption parameters of ZnO and Ag 0.2 @ZnO NC, nitrogen (N 2 ) adsorption-desorption isotherms at 77 K were recorded. Figure 10a shows the N 2 adsorption-desorption isotherms of ZnO and Ag 0.2 @ZnO NC. According to IUPAC classification, these curves obtained for evaluating surface area were approximately identical to that of Type IV isotherm with H 3 hysteresis loop 77 . The well-defined inflection around relative pressure (P/P 0 ) of 0.5-0.9 indicates the presence of a heterogeneously distributed mesoporous nature of particles 78 . The BET surface area (S BET ) was determined from isotherms using the BET equation 79 . The values of S BET were found to be 11.77 and 7.5 m 2 /g for ZnO and Ag 0.2 @ZnO NC, respectively, and the mesoporous material contains narrow pores that hinder the movement of nitrogen and limits the adsorption. The S BET of Ag 0.2 @ZnO NC decreased with the embedment of Ag NPs more than ZnO, revealing that the formed Ag NPs could have occupied and blocked the pores of ZnO. Figure 10b shows the pore size distribution curve obtained using the Barrett-Joyner-Halenda (BJH) model. It could be seen that most of the pores were in the size range of 2-40 nm, which provides evidence for the mesoporosity framework of ZnO and Ag 0.2 @ZnO NC. The BHJ average pore sizes of ZnO and Ag 0.2 @ZnO NC were 9.52 and 11.57 nm, respectively, and the calculated mean pore volumes were 0.027 and 0.023 cm 3 /g. The S BET , BHJ mean pore size, and pore volume of Ag 0.2 @ZnO NC were lower than that of ZnO because of the embedment of Ag NPs on the surface of ZnO.
Mechanism of photocatalytic activity. The schematic diagram of the photocatalytic degradation of dyes MB and CR by ZnO and Ag@ZnO NCs is proposed in Fig. 10c. The advanced oxidation processes (AOPs) generate ROS of highly reactive species such as superoxide anion radicals ( · O 2 ‾), and hydroxyl radicals ( · OH) are mainly involved in the degradation and mineralization of dyes into carbon dioxide (CO 2 ) and water 80 .
When ZnO is irradiated by the UV light of the simulated solar lamp, electrons in the valence band (VB) get excited to the conduction band (CB), leaving behind holes in the VB 81 . These photogenerated electrons get transferred to the Ag NPs as the CB energy level of ZnO is higher than the Fermi level (E FM ) of metallic Ag, www.nature.com/scientificreports/ which hinders the recombination and extends the lifetime of photogenerated (e‾/h + ) pairs, whereas Ag NPs in the NCs absorb visible light undergo surface plasmon resonance (SPR), and these excited electrons in the 3d orbit of Ag NPs get easily transferred to CB of ZnO owing to the interface effect of Ag/ZnO heterojunctions, yielding more superoxide anion radicals. The holes formed by the excitation of electrons will generate · OH radicals by oxidation of hydroxyl ions. Thus, the as-formed superoxide anion radicals and hydroxyl radicals are mainly responsible for the effective mineralization of dyes into CO 2 and water 82,83 . The increase of silver amount on the surface of ZnO decreases the photocatalytic degradation efficiency. The decrease in the photocatalytic degradation by Ag 0.4 @ZnO and Ag 0.8 @ZnO NCs could be due to the hindrance in the absorption of light by the excess of Ag NPs, which is in agreement with the PL results.

Applications of ZnO and Ag@ZnO NCs
Photocatalytic degradation of dyes. Photocatalysis happens on the surface of the photocatalyst, and the photocatalytic performance of ZnO was ameliorated by increasing the surface-to-volume ratio and by modifying the band structure by the incorporation of Ag NPs to improve the visible-light absorption properties and thereby efficiently restricting the recombination of photogenerated (e‾/h + ) pairs 21,84,85 . The photocatalytic properties of ZnO and Ag@ZnO NCs were evaluated via the degradation of dyes MB (cationic) and CR (anionic) under the simulated solar lamp. Figure 11a shows the UV-Vis absorption spectra of the degradation of MB with    Figure 11c shows the photocatalytic degradation (C t /C 0 ) as a function of time, where C t is the concentration of MB at the time "t", and C 0 is the initial concentration. The experimental solution containing the MB (1.0 mg/100 mL) and photocatalyst (0.1% w/v) was allowed for the adsorption-desorption equilibrium in the dark for 30 min, and the MB dye in the range of 3.0 ± 2.5-12.7 ± 4.2% was adsorbed on ZnO and Ag@ZnO NCs. The increase of silver content as Ag NPs on ZnO increased the adsorption of MB dye on its surface. Moreover, the strong MB dye adsorption capacities by Ag@NCs in the dark improved their photocatalytic performances in terms of their decolorization and degradation processes 86 . The degradation percentage of MB by ZnO, Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs were 89.4 ± 2.2, 99.2 ± 0.34, 97.6 ± 0.91, and 96.0 ± 0.1.5%, respectively after irradiation for 90 min (Fig. 11e). Ag 0.2 @ZnO as photocatalysts showed higher photocatalytic degradative activity than other NCs and ZnO, and it showed 100% degradation in 90 min. However, other Ag@ZnO NCs and ZnO showed ~ 100% photocatalytic degradation in 120 min. The higher photocatalytic activity by Ag 0.2 @ZnO NC suggests that adding silver to ZnO improves the photocatalytic activity significantly. Similarly, the UV-Vis absorption spectra of the degradation of CR dye with time by ZnO and different Ag@ ZnO NC were shown in Fig. 11b. The C t /C 0 degradation of CR versus time is shown in Fig. 11d. After incubation at dark for attaining adsorption-desorption equilibrium, CR dye of 26.0 ± 0.77-74.4 ± 1.3% was adsorbed onto ZnO and Ag@ZnO NCs. ZnO had a strong adsorption ability of CR on its surface; however, with an increase in the silver amount, the adsorption of CR on the surface of NCs decreases. Moreover, the strong adsorption capacities of ZnO and Ag@ZnO NCs in the dark improved their photocatalytic performances in their decolorization and degradation processes. The degradation percentages of CR by ZnO, Ag 0.2 @ZnO, Ag 0.4 @ZnO, and Ag 0.8 @ZnO NCs were 92.9 ± 0.5, 98.4 ± 2.4, 92.5 ± 1.5, and 86.1 ± 1.5%, respectively after irradiation for 55 min (Fig. 11f; Table 1). Analogous to MB degradation, the degradation of CR by Ag 0.2 @ZnO NC was higher than other Ag@ZnO NCs and ZnO. The photolysis of dyes without photocatalyst was also determined. Both dyes are barely degraded without photocatalyst, which indicates that both MB and CR dyes are stable in the aqueous environment under simulated solar irradiation. However, CR appears to be more stable than MB under the experimental conditions. There was photolysis of 26.97% and 4.26% for MB and CR dyes, respectively, after irradiation for 150 and 210 min ( Supplementary Figs. 2, 3). Among different metal/semiconductor NCs prepared using plant/fruit extracts, Ag@ZnO NCs prepared using GB extract as an additive yielded unique ellipsoidal morphology with mesoporosity, and the photodegradation efficiency of MB and CR was comparable to the previous reports ( Table 1). Reusability of the catalyst is one of the important intentions for photocatalytic reactions. The use of powdered catalysts has been limited due to the difficulties involved in the separation of catalysts after the degradation of pollutants. But, nowadays, present catalysts can be easily separable from the solution either by filtration or by centrifugation, and there is no permanent adsorption of dyes over the photocatalyst 87 . Regeneration of photocatalyst after every reaction was done by collecting the catalyst by centrifugation, washing with water, and calcination at 200 °C for 1 h. The photocatalytic activity of Ag 0.2 @ZnO NC remains intact for both MB and CR degradation for up to five adsorption/desorption cycles under selected conditions. There   Fig. 4). Thus, the recycling results reflect the commendable stability of both Ag NPs and ZnO structures in the Ag 0.2 @ZnO NC for the degradation of cationic dyes for wastewater treatment.
Antibacterial assay. Figure 12 shows the antibacterial activity of ZnO and other Ag@ZnO NCs against  www.nature.com/scientificreports/ extract-mediated synthesis of nano-ZnO against S. aureus, Serratia marcescens, Proteus mirabilis, and Citrobacter freundii. Even Ag NPs in Ag@ZnO NCs can cause membrane permeation and bacterial ROS production for the synergistic antibacterial activity with ZnO particles in the nanocomposite 91,92 . Zare et al. 93 evaluated the antibacterial potency of ZnO-Ag NC on bacteria. They proposed that physical interaction with bacterial cells causes disruption of cell membrane and oxidization of cell components for exhibiting broad-spectrum antibacterial activity against multidrug-resistant bacteria.

Conclusions
The synthesis of zinc oxide particles (ZnO) by direct precipitation method using goji berry extract as an additive and subsequent calcination in air promoted the formation of mesoporous ellipsoidal morphology with 0.59 µm (length), and 0.33 µm (width) was found to be hexagonal wurtzite crystal structure. The formation of silver nanoparticles on the surface of ZnO in the formation of Ag@ZnO nanocomposites using the GB extract provides a method of synthesizing highly porous metal/semiconductor NCs. The presence of polyphenols in the GB extract acts as both reducing and capping/stabilizing agents in preparing nanoparticles and/or nanocomposites. The asprepared Ag@ZnO NCs were characterized by several techniques, such as FT-IR, XRD, FE-SEM, TEM, EDS, XPS, and UV-Vis spectroscopy. The XRD analysis, and SEM-EDX and TEM micrographs confirmed the formation of Ag NPs on the surface of ZnO. The photocatalytic activity of Ag 0.2 @ZnO nanocomposite towards both MB and CR degradation in an aqueous medium was found to be higher than that of ZnO and other Ag@ZnO NCs at room temperature. Ag 0.2 @ZnO NC was photostable and reusable for cationic dyes even after five adsorption/ desorption cycles. The presence of Ag on the surface of ZnO promotes the separation of photogenerated charge carriers and enhances photocatalytic degradation of pollutants. In addition, they also showed good antibacterial activity against Staphylococcus aureus and Escherichia coli. Both, the photocatalytic and antibacterial activity of Ag 0.2 @ZnO were remarkably improved due to the generation of abundant ROS than that of ZnO particles and other Ag@ZnO NCs. This novel methodology utilizes fruit extract as a sustainable and eco-friendly additive to form the unique morphology of semiconductor particles, and as a reducing/stabilizing agent to form metal nanoparticles to prepare metal/semiconductor nanocomposites for wastewater treatment by photocatalysis and antimicrobial therapeutics.

Data availability
Data available on request from the authors.